Investments - Restricted Cash and Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Restricted Cash and Investments Items [Line Items]
Restricted cash	$ 190,470	$ 103,788
Restricted investments	4,440,935	4,295,008
Restricted cash and investments	4,631,405	4,398,796
Amortized cost	4,835,907	4,767,911
Third party agreements [Member]
Restricted Cash and Investments Items [Line Items]
Restricted cash	69,164	56,891
Restricted investments	1,283,941	1,299,569
Amortized cost	1,274,449	1,307,926
Related party agreements [Member]
Restricted Cash and Investments Items [Line Items]
Restricted cash	121,171	46,777
Restricted investments	1,992,095	2,225,066
Amortized cost	1,984,440	2,242,565
HTM related party agreements [Member]
Restricted Cash and Investments Items [Line Items]
Restricted investments	1,160,694	766,135
Amortized cost	1,132,916	752,212
US state regulatory authorities [Member]
Restricted Cash and Investments Items [Line Items]
Restricted cash	135	120
Restricted investments	4,205	4,238
Amortized cost	$ 4,067	$ 4,059